The advent of atmospheric pressure ionization (API) has resulted in an explosion in the use of LC/MS analysis. Various ion sources may be employed at API. For instance, there are currently four API techniques—electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photon ionization (APPI) and atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI). There is also a new concept of simultaneous ESI with APCI or APPI (multimode ionization). Each of these techniques share the common need for drying aerosol generated from the flowing liquid.
A number of approaches have been employed for drying aerosols. The two major approaches for drying aerosols have been the application of hot gas through convection or by hot surfaces by conduction.
Hot gas is the preferred approach for drying ESI aerosols, but at high liquid flow rates the amount of energy that can be delivered is very limited by the thermal capacity of the gas. The result is either a large volume of gas must be used or the gas must be heated to very high temperatures. Neither of these choices is particularly desirable since the gas used is expensive, high purity nitrogen. The other choice of heating the gas to very high temperatures seriously affects the materials that can be used in the source. Hot gas is not used for APPI or APCI sources because hot tubes are easy to install are more economical and analyte contact with the tube is permitted. Hot gas has been employed with AP-MALDI applications, but the methods are fairly crude and there is no real control of the gas to the ion source. In certain instances, the chambers are flooded with the heated gas to improve the overall instrument sensitivity. Ideally it would be desirable to be able to control the heating to improve overall ion cooling and ionization without ion clustering problems.
In addition, hot surfaces have also been employed for APPI, AP-MALDI, and APCI ion sources. In certain ion sources there are advantages of not having the aerosol come in contact with a surface. Many materials can be catalytic to chemical reactions. Avoiding surface contact can avoid or eliminate many of these problems. In addition there are subtle ionization mechanisms possible from gas shearing that are usable if the aerosol does not come into contact with a surface. For these reasons, there is a need for improvements over the presently existing devices and designs.
The practical problem with most of these techniques is temperature control. Many analytes are thermally sensitive and will not tolerate high temperatures. Uncontrolled temperatures make reliable analysis impractical. Small changes in solvent composition or flow rate can alter the ion source temperature. For this reason what is needed is a more controlled manner for temperature control.